Steel sheets having a galvanized layer containing magnesium (Mg) and aluminum (Al) may have excellent resistance to red rust corrosion, but since the exposed surface may contain mostly zinc (Zn) or zinc alloy (Zn alloy), white rust corrosion may occur on the surface when exposed to normal environmental conditions, and especially so in humid environments. Further, since magnesium and aluminum contained in a coating layer have better oxygen affinity than zinc, a blackening phenomenon is likely to occur when the amount of oxygen combining with zinc is insufficient.
Conventionally, as part of an anti-corrosive treatment, a metal surface may be pre-treated with chromate in an amount of 5 to 100 mg/m2 to form an organic coating. However, heavy metals such as chromium (Cr) or the like contained in pre-treating agents require additional pre-treatment equipment and processes, and moreover, the safety of steelworkers may be a concern due to heavy metal-containing waste water. Furthermore, manufacturing costs may increase since a hexavalent chromium-containing solution generated from washing water and waste water requires treatment using a special treatment process, and environmental pollution has been a serious problem due to a chromium ion being released from chromate-treated metal coated steel sheets when disposed or during use.
To overcome such problems as well as to secure corrosion resistance in the prior art, surface treatment agents such as an anti-corrosion metal coating agent free of chromium have been developed.
By way of example, Patent Document 1 relates to a surface treatment agent consisting of a zirconium carbonate, a vanadyl ion, and a zirconium compound, which is resistant to corrosion, but prone to blackening.
On the other hand, Patent Document 2 relates to a surface treatment agent consisting of a titanium-based, zirconium-based, phosphoric acid-based, molybdenum-based compound and the like, but which is unable to inhibit blackening in a hot-dip galvanized steel sheet using magnesium (Mg), aluminum (Al), or the like. In addition, Patent Document 3 relates to a surface treatment agent consisting of ammonium molybdate, aqueous dispersion polyurethane resins, isopropylamine, a zirconium ammonium carbonate, an epoxy-based silane coupling agent, and silica sol. However, in this case, due to increased thickness of the surface treatment film, determination of locations where conductivity and weldability are desired may be difficult, and when the thickness is reduced, resistance to corrosion may be insufficient.